The power consumption and performance of data centers are affected by the placement of data onto the hard drives of the data center. Several factors account for these effects, including data remanence, adjacent track erasure, power consumption in the event of a track refresh, and I/O throughput of different tracks on the hard drive.
Data remanence is an act of securely purging data such that there are no residuals of the data on the hard disk. Government regulations exist which mandate the need for secure deletion according to various standards. A hard disk drive includes one or more hard disks clamped to a rotating spindle and at least one head for reading or writing data on the disk. Storage on a hard disk is divided into “blocks”, which are in turn grouped into “tracks”. A disk head uses magnetism to read or write data onto the tracks. Data remanence involves the overwriting of blocks on a track by the disk head numerous times, depending on the standard being applied. Thus, data remanence involves multiple input/output (I/O) operations on the same blocks of a track.
As a track on the hard disk is written, adjacent tracks may be overwritten by the magnetic field generated in the disk head, and data recorded in the adjacent tracks may be erased or corrupted as a result. This phenomenon is called adjacent track erasure (ATE) or adjacent track interference (ATI) and are aggravated by the multiple overwrites required by secure deletion. In one approach to counter ATE or ATI, when a track has been overwritten a certain number of times, the adjacent tracks are read and rewritten, hence refreshing the data on the adjacent tracks. However, the I/O operations required in the refreshing of the adjacent tracks impose a performance penalty on the hard disk drive. Increasing the number of track refreshes also increases the power consumption of the hard drive.
The I/O throughput of different tracks of the hard drive affects the performance of the drive. Hard drives are fastest on the outer area of the drive. The outer area of the drive is about twice the circumference of the inner area, thus twice the data will pass under the drive's read/write heads with each rotation of the platter. The speed differential between the tracks in the outer area and the inner area of the drive leads to differentials in performance.
The challenge for the Information Technology (IT) owner is housing and operating the computational power of data centers for cloud computing. With more computational power in each unit volume, the industry is experiencing a significant increase in power density and hence a greater cooling challenge. The ability to tackle such computational tasks has driven energy costs higher.